This invention relates to an air pressure driven vacuum sewer system.
In a vacuum sewer system, the sewer pipe must be kept under partial vacuum to enable the waste transport, typical of a vacuum sewer system, to be accomplished. On the other hand, it is convenient to keep the sewage collecting container at atmospheric pressure, because this allows the container to be made to less demanding standards than if it were kept under partial vacuum and also facilitates the emptying of the container. The known solutions for achieving these two conditions are, however, relatively complicated and expensive. See, for instance, U.S. Pat. Nos. 3,629,099, 4,184,506, and 4,034,421.
It is known to use a liquid-driven ejector for generating vacuum in a vacuum sewer system. For example, U.S. Pat. No. 4,034,421 shows a system with a liquid driven ejector at the downstream end of the sewer, which ejector generates the partial vacuum necessary for sewage transport. However, this known arrangement is expensive because a separate circulation pump must be used to drive the ejector. Besides, the efficiency rate of the vacuum generation is low, typically only about 5 percent.
In the system shown in U.S. Pat. No. 4,034,421, the working medium supplied to the ejector is untreated sewage, which sets special demands, e.g. with regard to cleaning etc., on the circulation pump and on the ejector. Furthermore, although this sewage might have been ground, it is nevertheless inhomogeneous and therefore requires a large nozzle in the ejector and a high pumping rate and pressure. It would in principle be possible to use another liquid as working medium, but this has significant drawbacks, particularly when applied to a vacuum sewer system for a land-based passenger transport vehicle, such as a bus or a railroad train. In particular, use of another liquid as working medium would necessitate that a supply of liquid be carried aboard the vehicle. Further, it would be necessary either for the sewage collecting container to be sufficiently large to contain the liquid working medium as well as the sewage received from the waste receiving unit(s) or to provide a device for filtering liquid from the sewage downstream of the ejector, neither of which measures is attractive.
U.S. Pat. No. 4,791,688 shows a system that is similar to that shown in U.S. Pat. No. 4,034,421 but in which, in addition, there is employed an extra external air supply for ensuring sewage transport.